This invention relates generally to semiconductor device and integrated circuit fabrication, and more particularly the invention relates to high temperature contacts and interconnect structures in such devices and methods of making the same.
It is well known that a multilayer integrated circuit interconnection structure is desirable to improve circuit performance as well as to increase circuit density. Prior art structures include etched contact holes formed in an oxide coated wafer surface, a layer of aluminum formed on the oxide surface and in the contact holes to form contact with selected regions of the wafer. The aluminum layer is patterned to interconnect specific portions of the integrated circuit. This contact and interconnection structure has not been especially satisfactory for many applications because the highly reactive aluminum pits the silicon oxide during high temperature processing reducing the structure's breakdown voltage. Subsequent processing steps such as passivation and multilayer interconnection increases the reaction between the aluminum and the silicon oxide. Elevated temperature required for some processing steps, on the order of 900.degree. C., further promote aluminum and oxide reaction. The aluminum may recrystallize in formation which crack the insulating layer separating multiple metal layers. Thus, the use of aluminum contacts and interconnections severely restricts subsequent processing steps at elevated temperatures.
U.S. Pat. No. 4,265,935 issued to John H. Hall, applicant herein, for HIGH TEMPERATURE REFRACTORY METAL CONTACT ASSEMBLY AND MULTIPLE LAYER INTERCONNECT STRUCTURE discloses a much improved multilayer structure which withstands high temperature processing much better than aluminum. The structure includes a layer of refractory metal such as molybdenum between two layers of silicon. The bottom silicon layer provides an adherent interface with an underlying insulating layer such as silicon oxide, while the top silicon layer is provided so that ohmic contact can be easily made to the refractory metal. While the multilayer structure is relatively inert and withstands subsequent high temperature processing, the structure can develop a high contact resistance after being exposed to higher temperatures. This is due to the fact that silicon in the contact holes is extracted and purified at high temperatures by reaction with the molybdenum to raise its resistance. There is also a problem with residual silicon dioxide on the surface of the silicon substrate in the contacting areas which causes erratic contact resistance.
The present invention is directed to an improved high temperature contact and interconnect structure which overcomes problems associated with prior art structures.